Does Avast whitelist previously scanned files like Kaspersky and NOD32 do so the same file won’t be scanned again to speed things up after the first fufll scan?
Yes.
Yes and No.
Yes as in directly after a scan if there is any further activity.
No as in when there is a virus definitions update (or a reboot I believe)
Avasts uses A) Transient cache and B) a Persistent cache. See attached image.
Dave, the answer to the initial question is still, yes.
I still think it is to simple an answer, given the part of the quote.
[i][b]so the same file won't be scanned again[/b][/i] to speed things up after the first full scan?
So I expanded the answer and the user now has the full story.
Well, more (expanded) info never hurts… 
Awesome, just what I wanted!
Thanks
You’re welcome.
Just ensure this option is also enabled, avastUI > Scan > Scan for viruses - scroll down to Scan Settings at the bottom - Performance and ensure the Store data about scanned files in the persistent cache (see image). Whilst this is likely to slow the first few scans a little whilst this information is recorded it does speed up scans once the Persistent cache gets populated.
DavidR,
where does the persistent data reside in Windows XP? I just ran a quick scan after setting the above option,
and the place I thought it was kept, \documments and settings\all users\application data\avast software\persistent data
remains empty save for a few old logs.
Secondly, what is the point of the persistent cache when it is nullified after every avast update?
Transient cache in RAM makes more sense.
I suggest you look at my posts again as:
-
they don’t refer to any persistent cache log - I’m not even sure it would be wise to have such a log - that it is a file known only to avast and a user couldn’t manually add to the Persistent cache.
-
The Persistent cache isn’t nullified on a VPS update as shown in my first image.
Thnx David 
Bookmarked !
Greetz, Red.